1. Field of the Invention
This invention relates to a catalytic structure which can be effectively used for removing harmful substances such as carbon monoxide, nitrogen oxides and hydrocarbons contained in exhaust gas from automobiles and other sources of combustion gas discharge.
2. Background Art
Restrictions on release of harmful substances such as carbon monoxide, nitrogen oxides, hydrocarbons, etc., contained in exhaust gas from automobiles and various types of combustion apparatus have become increasingly strict in recent years. As an effective means for cleaning such exhaust gas, use is generally made of redox catalysts composed of noble metals.
Methods are khown for incorporating such noble metal catalysts in a carrier, as for instance disclosed in Japanese Patent Kokai (Laid-Open) No. 88848/80. According to the method of this patent, an activated alumina layer is formed on the surface of the three-dimensional network skeleton of a porous ceramic carrier, and said activated alumina layer is impregnated with an aqueous solution of a complex containing a noble metal for forming catalyst and dried, thereby obtaining a catalytic structure for cleaning exhaust gas, which structure has the catalyst uniformly carried in the surface of said three-dimensional network skeleton.
The noble metals such as platinum, rhodium, palladium, etc., usable as cleaning catalysts for such noxious matters as carbon monoxide, nitrogen oxides and hydrocarbons exist only scantly on the earth and are therefore very expensive.
The present inventors have noted in their studies that said conventional catalytic structure had the following problem. When exhaust gas is passed through the catalytic structure for treatment, the surface of the three-dimensional network skeleton of the catalytic structure presents two sides relative to the exhaust gas flow, that is, one side against which the incoming exhaust gas flow impinges and the other side against which the exhaust gas does not impinge, and naturally, the oxides of such substances as iron, sulfur, lead, etc., contained in exhaust gas tend to deposit on the side against which the exhaust gas flow impinges. Such deposits of oxides deteriorate the catalytic activity of said noble metal catalyst by poisoning or covering it. The catalyst carried on the side against which the exhaust gas flow impinges can perform its normal catalytic action in the early phase of use but loses its activity in long-time use, so that with said conventional catalytic structure, the costly noble metal catalyst could not be effectively utilized in long-time use.